Such a transfer gearbox is described in EP 2 821 672 A1 as a component of an electric drive unit. The electric drive unit has a rotor shaft which is the input shaft of a transmission device. The transmission device is formed from a differential gearbox and from a reduction gear stage. The reduction gear stage is a planetary drive, the sun gear of which sits on the rotor shaft. The differential is what is known as a spur gear differential in which the compensating gears are planetary gears and the output gears are sun gears. Each sun gear sits on an output shaft of the differential. The torques introduced via the input shaft/rotor shaft are input via the reduction gear stage into the differential and distributed via the differential to the output shafts and thus to driven vehicle wheels. The rotor shaft is a hollow shaft through which one of the output shafts is guided. An annular gap is formed between the rotor shaft and the output shaft. The output shaft and the respective sun gear are connected to one another in a rotationally fixed manner via a spline. The region of the electric machine between the stator and the rotor should, apart from wet-running electric motors, be kept free from lubricating oil. The transmission device is, however, lubricated with transmission oil. There are drive units in which the annular gap between the output shaft and the rotor shaft is used as an oil duct in order, for example, to supply lubricating oil to an anti-friction bearing which lies axially on the outside. There are, however, also drive units in which the anti-friction bearing which lies axially on the outside is lubricated in other ways or is lubricated over its lifespan.
For the sake of simplicity, the sun gears of differentials are generally provided with through-holes, so that the inner spline can be produced in an unhindered manner. The output shaft is provided with an outer spline which corresponds to the inner spline of the sun gear. This known plug-in connection is very easy to mount and is therefore very readily used. In terms of oil leaks, the plug-in connection between the sun gears of a differential is a weak point of the transfer gearbox since this is embodied with clearance. Lubricating oil can escape to the outside as a result of the leakage gaps associated with the clearance.